1,3-propanediol (PDO) is an industrially important chemical. PDO is used as a monomer unit to form polymers such as poly (trimethylene terephthalate) that are used in the production of carpets and textiles. PDO is also useful as an engine coolant, particularly in cooling systems that require coolants having low conductivity and low corrosivity.
PDO may be prepared industrially by hydroformylation of ethylene oxide under pressure in the presence of syngas (CO and H2) and a catalyst to prepare 3-hydroxypropionaldehye (HPA), followed by the hydrogenation of HPA to PDO. Preferred hydroformylation catalysts are metal carbonyls, especially cobalt carbonyl. The hydroformylation reaction is typically conducted in a solvent that is inert to the reactants, that will solubilize carbon monoxide and the catalyst, that is essentially immiscible in water, and that exhibits low to moderate polarity such that the hydroformylation product HPA can be extracted from the solvent with water. Such solvents include ethers such as methyl-t-butyl ether (MTBE), ethyl-t-butyl ether, diethyl ether, phenylisobutyl ether, ethyoxyethyl ether, diphenyl ether, and diisopropyl ether.
Following hydroformylation, the HPA is extracted and separated from the non-water miscible solvent with water. The separated aqueous solution of HPA is then hydrogenated to form PDO in the presence of a hydrogenation catalyst. The organic phase containing the hydroformylation reaction solvent and most of the metal carbonyl catalyst can be recycled to be reused in further hydroformylation.
The separated aqueous solution of HPA may be treated to remove species that can interfere with the performance of the hydrogenation catalyst in the conversion of HPA to PDO. The separated aqueous HPA solution typically contains from 4 to 60 wt. % HPA, residual syngas (including carbon monoxide), residual ethylene oxide, and residual metal carbonyl species from the hydroformylation reaction catalyst such as cobalt or rhodium carbonyl species including Co[Co(CO)4]2, Co2(CO)8, and Rh6(CO)16. Most hydrogenation catalysts are poisoned by carbon monoxide (CO) and the residual metal carbonyl species, so the aqueous solution of HPA is treated to remove these catalyst poisons from the solution. The aqueous solution of HPA may be degassed, oxidized and stripped, and then contacted with an acidic ion exchange resin to remove these hydrogenation catalyst poison sources.
The aqueous HPA solution may be degassed, oxidized, and stripped by passing an oxygen containing gas through a column or tank of the aqueous HPA solution—typically in a countercurrent flow arrangement. The pressure maintained on the aqueous HPA solution in the degasser-oxidizer-stripper column or tank is usually lower than the pressure in the hydroformylation reaction and extraction, so that CO gas is flashed from the aqueous HPA solution in the degasser-oxidizer-stripper column or tank, and any residual CO gas is stripped from the solution by the oxygen containing gas and any other stripping gas stream such as nitrogen.
The oxygen containing gas also oxidizes residual metal carbonyl species in the aqueous solution, ensuring that the metal species are water soluble, particularly in the presence of byproduct organic acids such as 3-hydroxypropionic acid in the aqueous HPA solution. The water soluble metal species are removed from the aqueous solution of HPA by contacting the solution with an acidic ion exchange resin.
The gaseous stream containing the degassed CO may be removed from the degasser-oxidizer-stripper tank or column to separate the CO from the aqueous HPA solution. The gaseous stream removed from the degasser-oxidizer-stripper is typically collected and condensed to recover any residual hydroformylation solvent contained therein. Recovery of the solvent from the gaseous stream is important to reduce environmental impact—for example, residual solvent MTBE cannot be vented directly to the atmosphere, as it is a regulated pollutant. The gaseous stream from the degasser-oxidizer-stripper may be compressed using a compressor, and the solvent may be recovered by chilling the compressed gas in a chilling tank.
It has been found that unexpected deposition of solids from the gaseous stream inhibits the efficient recovery of solvent from the gas phase. For example, where gas compression is used for solvent recovery solids deposit in the compressor, thereby fouling the compressor. As a result, the compressor works for only a short period of time before clogging with the deposited solids. The degasser-oxidizer-stripper and other portions of the process must then be shut down to clean or replace the compressor—which results in particularly poor process efficiency, especially in a continuous industrial operation. Further, the compressor suffers undue wear as a result of fouling by the deposited solids. For other solvent recovery configurations such as solvent recovery by liquid phase absorption, distillation, low temperature condensation, or solid adsorption, similar fouling may be observed due to the solids deposited from the gaseous stream.